Technique for forming laminated filamentary composites utilizing prestressing

ABSTRACT

Strips or sheets of material comprising filaments impregnated in a binder material are stressed in a stressing machine to a sufficient degree to cuase a number of the filaments to break. The strips or sheets are then layered and molded together to form a laminated composite. The composite material so formed is found to have significantly improved structural characteristics.

Elite Staes Crandall et a1.

TECHNIQUE FOR FORMING LAMINATED FILAMENTARY COMPOSITES UTILIZING PRESTRESSING Inventors: Walter E. Crandall, Malibu; George J. Mills, Santa Ana; Gary G. Brown, Diamond Bar, all of Calif.

Northrop Corporation, Los Angeles, Calif.

Filed: Aug. 22, 1973 Appl. N0.: 390,595

Related U.S. Application Data Continuation of Ser. No. 195,590, Nov. 4, 1971, abandoned.

Assignee:

U.S. Cl 156/180, 156/229, 156/243, 161/89,161/92, 264/138 Int. Cl B32b 31/16 Field of Search 156/180, 229, 243; 161/89, 161/90, 91, 92; 264/138 1 1 Feb. 4, 1975 [56] References Cited UNITED STATES PATENTS 3,127,306 3/1964 Turton et a1 161/89 3,283,389 11/1966 Nisbet et a1 161/89 X 3,395,744 8/1968 Wolf et a1 139/420 X 3,674,581 7/1972 Kalnin et a1. 156/180 3,682,731 8/1972 Moraney 156/229 X Primary Examiner-Edward G. Whitby Attorney, Agent, or Firm-Edward A. Sokolski [57] ABSTRACT Strips or sheets of material comprising filaments impregnated in a binder material are stressed in a stressing machine to a sufficient degree to cuase a number of the filaments to break. The strips or sheets are then layered and molded together to form a laminated composite. The composite material so formed is found to have significantly improved structural characterisnos.

15 Claims, 2 Drawing Figures PATENTED 3. 864. 1 84 FIG.2

TECHNIQUE FOR FORMING LAMINATED FILAMENTARY COMPOSITES UTILIZING PRESTRESSING This is a Continuation of Application Ser. No. 195,590, filed Nov. 4, 1971 now abandoned.

This invention relates to the fabrication of laminated filamentary composites, and more particularly to such a technique involving the prestressing of the individual layers of the laminate to induce breaking of the weaker filaments thereof prior to the molding of the composite.

Laminated filamentary composites utilizing filaments of material such as boron, quartz, fiberglass, graphite, etc., molded in a binder of a suitable resin, are used extensively in forming high strength to weight ratio composites which are found to have excellent structural characteristics. To improve such characteristics, prestressing of the filaments prior to their binding to the matrix has been suggested to provide a higher usable material strength or a greater material stiffness. In the prior art approaches along these lines, such stressing has been done to provide pre-loading of the filaments in tension to counterbalance compression in the binder matrix. The technique of this invention involves a somewhat different approach, wherein stressing is done to an extent necessary to break the weaker areas of filaments prior to the formation of the laminate. This it has been found, rather than weakening the composite as one would initially expect to occur, rather unexpectedly provides a significantly strengthened end product. This is believed to be the case because the broken filaments are randomly distributed in the laminate and supported by the binder in such a manner that they do not contribute to the weakening of the overall structure. On the other hand, such weaker filaments when not broken in accordance with the technique of the invention provide weak links in the laminated composite which tend to break before the stronger filaments. Such breakage in the composite contributes materially to the overally fracture of adjacent strong fibers and also rupture of the binder material. By prebreaking the filaments such weaknesses in the composite are significantly reduced.

It is therefore an object of this invention to improve the structural characteristics of laminate composites formed from filaments impregnated in a binder material.

It is a further object of this invention to improve the strength of a laminated filamentary composite by breaking the weaker filaments of the composite prior to the forming of the laminates.

Other objects of this invention will become apparent as the description proceeds in connection with the accompanying drawings, of which:

FIG. I is a schematic drawing illustrating a method for stressing the filaments in carrying out the technique of the invention, and

FIG. 2 is a drawing illustrating the molding together of prestressed layers forming the composite by the technique of this invention.

Briefly described, the technique of the invention involves the stressing of strips or sheets of a material comprising filaments impregnated in a matrix binder. Typically, the filamentary material may be of boron, quartz, fiberglass or graphite, while the impregnate may be ofa plastic resin such as a suitable epoxy resin. The strips or sheets of the material are stressed until a number of the filaments thereof break. A plurality of strips are then layered against each other, these layers being molded together between a press by the application of heat thereto. This results in a laminated composite material having improved structural characteristics.

Referring now to the figures, the laminated composite is fabricated in the following manner: First, the tape 11 which is to be used to form layers in the laminate is placed around stressing rollers 13 and 14, the tape being fed from feed spool 16 to take-up spool 17 through guide spool 18, as shown in the drawing. A weight 15 is placed on spool 18 to tension the tape on the rollers and thereby stress the material as it passes over the roll radius. The stressing may be controlled by varying the roll radius and changing the weight. Typically, material 11 may comprise boron filaments which have been impregnated with a binder material such as a suitable epoxy resin. A typical commercially available type which may be used is SP272 Boron Epoxy Tape which is available commercially from Minnesota Mining & Manufacturing Co. Other suitable tapes which may be utilized include tapes utilizing quartz, fiberglass or graphite filaments impregnated with an appropriate resin binder. Sufficient weight 15 is used so as to stress the tape enough to break a number of the filaments thereof. The number of filaments actually broken will vary in individual cases and in some instances could be as few as 10% of the total number and in other instances over 50%.

After the tape to be used in fabricating a laminate has been stressed as just described, the tape is cut into strips which are laid against each other as shown in FIG. 2 in a mold l9 and pressed together with pressures of between about 15 and psi being applied. While the pressure is being applied, heat is applied at a temperature of 280350F. The part is then cured under pressure for l 2 hours at a temperature of about 350F to produce the laminated composite 20 shown in FIG. 2.

The following are typical specific examples of the utilization of the technique of the invention.

EXAMPLE I A one-inch wide tape of Boron Epoxy Type SP272, as mentioned above, was stressed in the machine shown in FIG. 1. The rollers 13 and 14 had a diameter of 0.392 inches and the load 15 had a weight of l 1.5 lbs. Nine strips of tape were cut from the stressed tape and these layered against each other as shown in FIG. 2 between two sheets of porous Armalon, a fiber reinforced Teflon material manufactured by Dupont de Nemours & Co., a mold being formed by the Armalon and tape placed along the ends of the strips of material. The layered material was then placed in a hot press at a temperature of 280F and pressed for 10 minutes at 15 psi. After ten minutes, the pressure was increased to 75 psi and the temperature raised to 350F, the part being cured at this temperature and pressure for 1% hours. It is to be noted that the tape was examined prior to the layering thereof and it was noted that about half the filaments had been broken in the stressing operation.

EXAMPLE II A tape of Boron Epoxy SP272 was stressed in a machine as shown in FIG. 1, having rollers 13 and 14, with a 0.3 inch diameter and with weight 15 being 15 pounds, until approximately 45% of the filaments were broken. A nineply composite was then fabricated by placing nine sections of the tape against each otheras shown in FIG. 5, between porous Armalon sheets. This part was placed in a press with 20 psi platen pressure being applied for five minutes at a temperature of 300F. The part was then cured by applying 75 psi thereto at a temperature of 350F for a period of 2 hours.

EXAMPLE lll A 3 inch wide tape of graphite epoxy Type A, 46l7/MPDA, available from Fatheringel & Harvey, England, was stressed in the machine shown in FIG. I. The rollers 13 and 14 had a diameter of 0.250 inches and the load 15 had a weight of 40 pounds. Nine strips of tape were cut from the stressed tape and these layered against each other as shown in FIG. 2 between two sheets of porous Armalon. The layered material was then autoclave cured for 2 hours at 350F and lOO psi.

EXAMPLE IV A one inch wide tape of Boron Polyimide, Monsanto 704 available from Monsanto Co., was stressed longitudinally with sufficient stress to break 12 percent of the existing filaments. Eight strips of tape were cut from the stressed tape and cured at 360F for 10 minutes under contact pressure of psi. After minutes, the temperature was increased to 550F with l00 psi. These conditions were maintained for 1 hour. The press was then cooled to 150F, pressure was released, and the mold was removed from the press.

These are but four examples of the use of the technique of the invention and it should be obvious that it can be applied to many different types of filamented material in a great variety of manners to achieve the desired end result.

The technique of this invention thus enables the fabrication of filamented or fibered composites having substantially improved structural characteristics over those of the prior art. Test data indicates that significant improvement in tensile strength is achieved in composites processed in accordance with the techniques of this invention as compared with similar composites not so processed.

While the technique of this invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the following claims.

We claim:

1. A technique for fabricating fibered laminated composites from a plurality of sections formed of similar type of load bearing filaments impreganted with a matrix material comprising the steps of:

stressing each of said sections until a number of the filaments thereof are broken,

layering said sections against each other, and

molding said layered sections into a laminated composite.

2. The process of claim 1 wherein said sections are molded into a composite by simultaneously applying heat and pressure to the layered structure.

3. The technique of claim 1 wherein said layered sections comprise boron filaments impregnated with epoxy resin.

4. The technique of claim 1 wherein said layered sections comprise boron filaments impregnated with polyimide resin.

5. The process of claim 1 wherein said sections are stressed until approximately half the filaments thereof are broken.

6. The technique of claim 1 wherein said layered sections comprise graphite filaments impregnated with epoxy resin.

7. The technique of claim 1 wherein said sections are cut from a tape, said tape being stressed in a stressing machine over rollers having a predetermined radius by the application of a predetermined load to said tape.

8. The technique of claim 1 wherein said sections are stressed until approximately 12 percent of the filaments thereof are broken.

9. A technique for fabricating sections formed of similarly type of load bearing filaments impregnated with a matrix material for use in making a laminated composite formed from a plurality of such sections layered against each other and molded together comprising:

stressing each of said sections and all of the filaments thereof uniformly with a predetermined force such as to break a number of the weaker filaments and to leave the stronger filaments unbroken.

10. The technique of claim 9 wherein said layered sections comprise boron filaments impregnated with epoxy resin.

11. The technique of claim 9 wherein said layered sections comprise boron filaments impregnated with polyimide resin.

12. The process of claim 9 wherein said sections are stressed until approximately half the filaments thereof are broken.

13. The technique of claim 9 wherein said layered sections comprise graphite filaments impregnated with epoxy resin.

14. The technique of claim 9 wherein said sections are cut from a tape, said tape being stressed in a stressing machine over rollers having a predetermined radius by the application ofa predetermined load to said tape.

15. The technique of claim 9 wherein said sections are stressed until approximately 12 percent of the filaments thereof are broken. 

1. A TECHNIQUE FOR FABRICATING FIBERED LAMINATED COMPOSITES FROM A PLURALITY OF SECTIONS FORMED OF SIMILAR TYPE OF LOAD BEARING FILAMENTS IMPREGANTED WITH A MATRIX MATERIAL COMPRISING THE STEPS OF: STRESSING EACH OF SAID SECTIONS UNTIL A NUMBER OF THE FILAMENTS THEREOF ARE BROKEN, LAYERING SAID SECTIONS AGAINST EACH OTHER, AND MOLDING SAID LAYERED SECTIONS INTO A LAMINATED COMPOSITE.
 2. The process of claim 1 wherein said sections are molded into a composite by simultaneously applying heat and pressure to the layered structure.
 3. The technique of claim 1 wherein said layered sections comprise boron filaments impregnated with epoxy resin.
 4. The technique of claim 1 wherein said layered sections comprise boron filaments impregnated with polyimide resin.
 5. The process of claim 1 wherein said sections are stressed until approximately half the filaments thereof are broken.
 6. The technique of claim 1 wherein said layered sections comprise graphite filaments impregnated with epoxy resin.
 7. The technique of claim 1 wherein said sections are cut from a tape, said tape being stressed in a stressing machine over rollers having a predetermined radius by the application of a predetermined load to said tape.
 8. The technique of claim 1 wherein said sections are stressed until approximately 12 percent of the filaments thereof are broken.
 9. A TECHNIQUE FOR FABRICATING SECTIONS FORMED OF SIMILARLY TYPE OF LOAD BEARING FILAMENTS IMPREGNATED WITH A MATRIX MATERIAL FOR USE IN MAKING A LAMINATED COMPOSITE FORMED FROM A PLURALITY OF SUCH SECTIONS LAYERED AGAINST EACH OTHER AND MOLDED TOGETHER COMPRISING: STRESSING EACH OF SAID SECTIONS AND ALL OF THE FILAMENTS THEREOF UNIFORMLY WITH A PREDETERMINED FORCE SUCH AS TO BREAK A NUMBER OF THE WEAKER FILAMENTS AND TO LEAVE THE STRONGER FILAMENTS UNBROKEN.
 10. The technique of claim 9 wherein said layered sections comprise boron filaments impregnated with epoxy resin.
 11. The technique of claim 9 wherein said layered sections comprise boron filaments impregnated with polyimide resin.
 12. The process of claim 9 wherein said sections are stressed until approximately half the filaments thereof are broken.
 13. The technique of claim 9 wherein said layered sections comprise graphite filaments impregnated with epoxy resin.
 14. The technique of claim 9 wherein said sections are cut from a tape, said tape being stressed in a stressing machine over rollers having a predetermined radius by the application of a predetermined load to said tape.
 15. The technique of claim 9 wherein said sections are stressed until approximately 12 percent of the filaments thereof are broken. 